72O 
AN ELECTRONIC PHOTORECEPTOR 
SENSITIVE TO SMALL CHANGES IN INTENSITY 
T. Delbrck and C. A. Mead 
256-80 Computer Science 
California Institute of Technology 
Pasadena, CA 91125 
ABSTRACT 
We describe an electronic photoreceptor circuit that is sensitive to 
small changes in incident light intensity. The sensitivity to changes 
in the intensity is achieved by feeding back to the input a filtered 
version of the output. The feedback loop includes a hysteretic el- 
ement. The circuit behaves in a manner reminiscent of the gain 
control properties and temporal responses of a variety of retinal 
cells, particularly retinal bipolar cells. We compare the thresholds 
for detection of intensity increments by a human and by the cir- 
cuit. Both obey Weber's law and for both the temporal contrast 
sensitivities are nearly identical. 
We previously described an electronic photoreceptor that outputs a voltage that is 
logarithmic in the light intensity (Mead, 1985). This report describes an extension 
of this circuit which was based on a suggestion by Frank Werblin that biological 
retinas may achieve greater sensitivity to changes in the illumination by feeding 
back a filtered version of the output. 
OPERATION OF THE CIRCUIT 
The circuit (Figure 1) consists of a phototransistor (P), exponential feedback to 
P (Q1, Q2, and Q3), a transconductance amplifier (A), and the hysteretic element 
(Q4 and Q5). In general terms the operation of the circuit consists of two stages of 
amplification with hysteresis in the feedback loop. The light falls on the parasitic 
bipolar transistor P. (The rest of the circuit is shielded by metal.) P's collector 
is the substrate and the base is an isolated well. P and Q form the first stage of 
amplification. The light produces a base current IB for P. The emitter current Is 
is 19I, neglecting collector resistance for now. /9 is typically a few hundred. The 
feedback current IQ is set by the gate voltage on Q/Q2, which is set by the current 
through Q3, which is set by the feedback voltage Vlb. In equilibrium Vlb will be 
such that IQ = Is and some voltage Vt, will be the output of the first stage. The 
An Electronic Photoreceptor Sensitive to Small Changes 721 
negative feedback through the transconductance amplifier A will make V,  Vfb. 
This voltage is logarithmic in the light intensity, since in subthreshold operation 
the currents through Q2 and Q3 are exponential in their gate to source voltages. 
The DC output of the circuit will be Vou  V.b = Vdd -- (2kT/q) log It, neglecting 
the back-gate effect for Q2. Figure 5a (DC output) shows that the assumption of 
subthreshold operation is valid over about 4 orders of magnitude. 
Q - 
Vou 
c 
Figure 1. The photoreceptor circuit. 
Now what happens when the intensity increases a bit? Figure 2a shows the current 
through P and Q as a function of the voltage V,. Both P and Q act like current 
sources in parallel with a resistance, where the value of the current is set, respec- 
tively, by the light intensity and by the feedback voltage V/b. When the intensity 
increases a bit the immediate result is that the curve labeled Ir in Figure 2a will 
shift upwards a little to the curve labeled I. But IQ won't change right away - 
it is set by the delayed feedback. The effect on V, will be that V, will drop by the 
amount of the shift in the intersection of the curves in Figure 2a, to Vl. Because 
interesting gain control properties arise here we will analyze this before going on 
with the rest of the circuit. 
In Figure 2b, we model P and Q as current sources with associated drain/collector 
resistances. Now, 
rp + rQx 
722 Delbrilck and Mead 
r, and rQ physically arise from the familiar Early effect, a variation of depletion 
region thickness causing a variation in the channel length or base thickness. It is 
a reasonable approximation to model the drain or collector resistance due to the 
Early effect as r = Ve/I, where Ve is the Early voltage and is typically tens of volts, 
and I is the value of the current source. 
Figure 2. The first stage of amplification. a: The curves show the current through 
the phototransistor P and feedback transistor Q as a function of the voltage V,. 
Since I, = IQ the intersection gives the voltage V/,. b: An equivalent circuit model 
for these transistors in the linear region. The Early effect leads to drain/collector 
resistances inversely proportional to the value of the current source. 
Substituting this approximation for r, and r into the above expression for 
and letting 5I = 5IB, we obtain 
= 
where V,e and V, are the Early voltages associated with the phototransistor and 
the feedback transistor Q, respectively. In other words, the change in Ve is just 
proportional to the 'contrast'I/I. Figure 4a shows test results which support 
this model. 
A detector which encodes the intensity logarithmically (so that the output V in 
response to an input I is V = log I) would also give 5V = I/I. Our in our circuit 
the gain control properties for transients arise from an unrelated property of the 
conductances of the sensor and the feedback element. Comparing the gains for DC 
and for transients in our circuit and using the expression for the DC output given 
earlier, we find that the ratio of the gains is 
transient gain Ve,, 
  200 
DC gain 2kT/q 
An Electronic Photoreceptor Sensitive to Small Changes 723 
assuming V,rlIV,, = 10V and kT/q = 25mV. 
Finally, let us consider the operation of the rest of the circuit. The second stage of 
amplification is done by the transconductance amplifier A. A produces a current 
which is pr,o, portional to the tanh of the difference between the two inputs, I -- 
G tanh( 2kT/q I' When the output of the amplifier is taken as a voltage, the voltage 
gain is typically a few hundred. Following the transconductance amplifier there is 
a pair of diode connected transistors, Q4 and Q5 (Figure 3a}, which we call the 
hysteretic element. This pair of transistors has an I-V characteristic which is similar 
to that of Figure 3. The hysteretic element conducts very little until the voltage 
across it becomes substantial. Thus the transconductance amplifier works in a dual 
voltage-output/current-output mode. Small changes in the output voltage result in 
little change in the feedback voltage. Larger changes in the output voltage cause 
current to flow through the hysteretic element, completing the feedback loop. This 
represents a form of memory, or hysteresis, for the past state of the output and a 
sensitivity to small changes in the input around the past history of the input. 
I v I 
a b 
Figure 3. a: The hysteretic element. b: I- V characteristic. 
COMPARISON OF CIRCUIT AND RETINAL CELLS 
We felt that since the circuit was motivated by biology it might be interesting to 
compare the operational characteristics of the circuit and of retinal cells. Since the 
circuit has no spatial extent it cannot model any of the spatially mediated effects 
(such as center-surround) seen in retinas. Nonetheless we had hoped to capture 
some of the temporal effects seen in retinal cells. In Figure 4 we compare the 
responses of the circuit and responses of a retinal bipolar cell to diffuse flashes of 
light. The circuit has response characteristics closest to those of retinal bipolar 
The circuit's gain control properties are very similar to those of bipolar cells. Figure 
5 shows that both the circuit and bipolar cells tend to control their gain so that 
they maintain a constant output amplitude for a given change in the log of the 
intensity. 
724 DelbrQck and Mead 
The response characteristics of the circuit differ from those of bipolar cells in the 
following ways. First, the gain of the circuit for transients is much larger than that 
of the bipolar cell, as can be seen in Figure 5, with concomitantly much smaller 
dynamic range. The dynamic range of the bipolar cell is about 1.5 - 2 log units 
around the steady intensity, while for the circuit the dynamic range is only about 
0.1 log unit. 
Input 2.5 BACKGROUND 
.0 background i\ 
' 
0.2ms 
(2 decades attenuated) 
15 
20m Noe level 
I 
a b 
mV 
Figure 4. The responses of the circuit compared with responses of a retinal bipolar 
cell. a: The output of the circuit in response to changes in the intensity. The 
background levels refer to the same scale as shown in Figure 5. The bottom curve 
shows the noise level; from this one can see that a detection criterion of signal/noise 
ratio equals 2 is satisfied for increments of 1- 2%, in agreement with Figure 6. Note 
that a 2 decade attenuation hardly changes the response amplitude but the time 
constant increases by a factor of a hundred. b: The response of a bipolar cell (from 
Werblin, 1974). The numbers next to the responses are the log of the intensity of 
the flash substituted for the intial value of the intensity. Note the bidirectionality 
of the response compared to the circuit. 
An Electronic Photoreceptor Sensitive to Small Changes 725 
log(Intensity) 
(mV) - 
 4 
.4  
log(Intensity) 
b 
Figure 5. The operating curves of the circuit (a) compared with retinal bipolar 
cells (b) (adapted from Werblln, 1974). The curves show the height of the peak of 
the response to flashes substituted for the initial intensity. The initial intensity is 
given by the intersection of the curves with the abscissa. Note the difference of the 
gain and the dynamic range. The squares show the DC responses. The slope of 
the DC response for the circuit is less than expected, probably because there is a 
leakage current through the hysteretic element. 
Second, the response of bipolar cells is symmetrical for increases and decreases in 
the intensity. This can probably be trced to the symmetrical responses of the cones 
from which they receive direct input. The circuit, on the other hand, only responds 
strongly to increases in the light intensity. The response in our circuit only becomes 
symmetrical for output voltage swings comparable to kT/q, probably because the 
limiting process is recombination in the base of the phototransistor. 
Third, the control of time constants is dramatically different. In Figure 4a the top 
set of responses is on a time scale 100 times expanded relative to the bottom scale. 
The circuit's time constant, in other words, is roughly inversely proportional to the 
light intensity. This is not the case for bipolar cells. Although we do not show 
it here, the time constant of the responses of bipolar cells hardly varies with light 
intensity over at least 4 orders of magnitude (Werblln, 1974). 
The circuit's action differs much more from that of photoreceptors, amacrine, or 
ganglion cells. Cones show a much larger sustained response relative to their tran- 
sient response. Amacrine and ganglion cells spike; our circuit does not. And the 
circuit differs from on/off amacrine and ganglion cells in the asymmetry of its re- 
sponse to increases and decreases in light intensity. 
726 Delbrilck and Mead 
EYE rs. CHIP 
We compared the sensitivity to small changes in the light intensity for one of us and 
for the circuit in order to get an idea of the performance of the circuit relative to a 
subjective scale. The thresholds for detection of intensity increments are shown in 
Figure 6. 
10' 
9 
8 
log(Increment) 7 
6 
5 
Human 1.6% 
Photoreceptor 
I I I I I I 
6 7 8 9 10 11 
log(Xbsorbable uanta/sec) 
Figure 6. Thresholds for detection of flicker 
The subject (TD) sat in a darkened room foveating a flickering yellow (583nm) LED 
at a distance of 75cm. The LED subtended 22' of arc and the frequency of flicker 
was 5 Hz. Threshold determination was made by a series of trials, indicated by a 
buzzer, in which the computer either caused the LED to flicker or not to flicker. 
The percentage of flicker was started at some large amount for which determination 
was unambiguous. For each trial the subject pressed a button if he thought he saw 
the LED flickering. A incorrect response would cause the percentage of flicker for 
the next trial to be increased. A correct response would cause the percentage of 
flicker to be decreased with a probability of 044. The result after a hundred trials 
would be a curve of percentage flicker vs. trial number which started high and then 
leveled off around the 75% correct level, taken to be the threshold (Levitt, 1971). 
The threshold for the circuit was determined by shining the same LED onto the chip 
directly. The threshold was defined as the smallest amount of flicker that would 
result in a signal-to-noise ratio of 2 for the output. The two sets of thresholds were 
scaled relative to each other using a Tektronics photometer and were both scaled to 
read in terms of absorbable quanta, defined for the human as the number of photons 
hitting the cornea and for the circuit as the number of quanta that hit the area of 
the phototransistor. There is a bias here favoring the circuit, since the other parts 
An Electronic Photoreceptor Sensitive to Small Changes 727 
of the circuit have not been included in this area. Including the rest of the circuit 
would raise the thresholds for the circuit by a factor of about 3. 
The results show that both the circuit and the human approximately obey Weber's 
law (I/I at threshold is a constant), and the sensitivities are nearly the same. The 
highest sensitivity for the circuit, measured at an incident intensity of 660W/cm 2, 
was 1.2%. This is about half the intensity in a brightly lit office. 
APPLICATIONS 
We are trying to develop these sensors for use in neurophysiological optical dye 
recording. These experiments require a sensor capable of recording changes in the 
incident intensity of about 1 part in 103-4 (Grinraid, 1985). The current technique 
is to use integrated arrays of photodiodes each with a dedicated rack-mounted low 
noise amplifier. We will try to replace this arrangement with arrays of receptors 
of the type discussed in this paper. Currently we are 1 to 2 orders of magnitude 
short of the required sensitivity, but we hope to improve the performance by using 
hybrid bipolar/FET technology. 
CONCLUSION 
This circuit represents an example of an idea from biology directly and simply 
synthesised in silicon. The resulting circuit incorporated not only the intended 
idea, sensitivity to changes in illumination, but also gave rise to an unexpected gain 
control mechanism unrelated to exponential feedback. The circuit differs in several 
ways from its possible biological analogy but remains an interesting and potentially 
useful device. 
Acknowledgements 
This work was supported by the System Development Foundation and the Office of 
Naval Research. Chips were fabricated through the MOSIS foundry. 
We thank Prank Werblin for helpful comments and Mary Ann Maher for editorial 
assistance. 
References 
A. Grinraid. (1OSS) Real time optical ma,ping of neuronal activity: from single 
growth cones to the intact mammalian brain. Ana. Rev. NeuroseS. 8:263-305. 
H. Levitt. (1971) Transformed up-down methods in psychoacoustics. J. Aoust. So. 
Am. 49:467-477. 
C. Mead. (1985) A Sensitive Electronic Photoreceptor. In 1985 Chapel Hill Coafer- 
eace oa VLSI. 463-471. 
F. Werblb. (1974) Control of Retinal Sensitivity II: Lateral Interactions at the 
Outer Plexiform Layer. J. Phziolog. 68:62-87. 
